Finally got to stretch my rifle's legs yesterday.  Rifle is a 24" FN SPR A1 .308 with a 1:12 twist.  The projectile I use is 168gr Hornady BTHPM at 2625 FPS. We were shooting 12"x15" gongs at 300, 400, 530 and 630 yards.  Warmed up with a few 300s, dialed in 400 and wore that gong out--almost couldn't miss.  Moved over to the 530, dialed in 13 MOA and hit first try--added another 1/2 MOA and hit 35 out of 40--had some horizontal dispersion due to wind and my misses were impacting just to the left when a gust caught me, but I kept most in a 4" group.  That's roughly .7 MOA and exactly what I expected to see.


Went out to the 630 yard target, dialed in 18.75 MOA and got a 50% hit rate.  My impacts were all over the target.  Top/bottom/left/right/center with no trend or pattern.  My misses were over/under/left/right.  No trend or pattern. I took my rifle back to the 530, dialed back to 13.5 MOA and shot 10 for 10, group nicely centered.  Went back to 630 and without changing my turrets, I held over 5 MOA and shot 5 more.  I missed all 5.  Impacts were all around the target again.  Over/under/left/right.  

There was another shooter there who was connecting more often than not at 630.  His impacts were holding about 5" vertical and he was seeing the same left wind drift.  I had him shoot my rifle at 630 and had the same problems I was having.  Missed 3 out of 5, right/under/over with no pattern.  Hits were high center and low right. I shot his rifle and hit 4 out of 5, all within a 6" vertical spread with left HD.  My miss was due to a gust that started as the trigger broke and it impacted about a foot left.  



Has anyone else ever seen something like this?  I'm thinking that my load is destabilizing just past 530--going from .7 MOA to 3+ MOA, although with the speeds I'm getting, I thought I could get it to 800 yds before it upset.  Is there a more optimal bullet for a 1:12 twist?

Not knowing whether or not you're shooting off the bipod the next question I'd have is how many rounds do you have from that barrel?

You seem to be assuming this is easy--it is not::

a) 630 is roughly 20% farther than 530.
b) 18 MoA of drop is 50% more than 13 MoA of drop.
c) the wind dials will be another 20%
d) you did not describe any upwards or downwards slope difference--so let's ignore that.

So 630 yards is roughly 1.2×1.5×1.2 = 2.16× harder than 530.

As you get out towards 800 the BC of the 168s start to come into their own (i.e., start being a problem).

I have seen something like that before.
What was the round count since the last good barrel cleaning?
What range elevation are you at?
Are you shooting from bag or bipod?
Inquiring minds want to know.....

Yes a 185 Berger Juggernaut. They worked great to 1k on cold days at sea level out of my 20" 12twist.

If you want save some money try the 175 Nosler Custom Comp...or 178eldm


Pardon the quality. Your loads.

Probability at 530yds
Attached File


630yds
Attached File


630 @3moa
Attached File




185 juggernaut @2585fps

630yds
Attached File


530 yds 185
Attached File

This one looks pretty simple, actually.  At 500 yards, that bullet is entering the trans-sonic velocity range.  Bullets flying in the velocity range of about 1200 to 1050 of so FPS lose stability rapidly.  Aircraft experience a similar phenomenon - they can be super stable at either subsonic or supersonic speeds, but the transonic region took a lot of R&D to work around.  Basically, you're shooting outside the effective range of that load, and might want to look at, say a VLD bullet design or bump up to a 180 grain SMK, or switch to a .300 Mag of some sort for those distances.